This invention relates to a safety circuit in the automatic transmission control system of a vehicle. More particularly, the invention relates to a safety circuit for use with an automatic transmission control system of the type having a control circuit which receives a first input signal responsive to the vehicular velocity and a second input signal with which the first input signal is compared to determine a speed change point, and which then produces a speed change or shifting control signal that is delivered to a transmission shifting control circuit; and the safety circuit operates in response to a failure such as interruption or loss of the signal responsive to the vehicular velocity.
Conventional automatic transmission control systems come equipped with a control circuit which receives a signal responsive to the vehicular velocity as well as a signal corresponding to the output torque of the engine or a signal responsive to the size of the throttle opening, and which, upon comparing the two input signals, produces a shifting control signal that causes the vehicle transmission to shift the transmission ratio ("gear ratio" hereinafter). The signal responsive to the velocity of the vehicle may be supplied by a well-known reed switch type of vehicular velocity sensor which is adapted to sense the rotational speed (rpm) of the drive shaft. This type of vehicular velocity sensor has a constant voltage applied across both its terminals and is opened and closed by a magnet that rotates in accordance, with the revolution of the drive shaft, thereby producing constant voltage pulses at a frequency which is in accordance with the drive shaft rpm. Ordinarily, the vehicular velocity sensor is incorporated within a speedometer, and is adapted to sense the rotational speed of the transmission output shaft as the corresponding rotational speed of the speedometer cable, and to produce the vehicular velocity responsive signal, in the form of the constant-voltage pulses, at the same time that the speedometer indicates the velocity of the vehicle.
Failures which can develop in the foregoing system for sensing and transmitting the signal responsive to vehicular velocity include breakage of the speedometer cable, a malfunction in the gear train for transmitting the rotational speed, a malfunction in the vehicular velocity sensing reed switch per se, or severance of the wiring that carries the output signal from the vehicular velocity sensor. In general, failures of the aforementioned type bring about an interruption in the vehicular velocity responsive signal. In such cases, the output signal acquired from the velocity sensor ordinarily is similar to that obtained when the vehicle is at rest. For example, in a case where one end of the reed switch is connected to body ground and the other end is connected to a positive constant-voltage line, a failure in the reed switch gives rise to a discontinuity in the train of constant-voltage pulses, i.e., a continuous non-pulse signal, and the resulting output signal resembles that produced when the vehicle is at rest. The same signal will be produced as a result of any of the failures described above. The control circuit of the automatic transmission control system would interpret such a signal as indicating that the vehicle is at rest and would send a control signal to the transmission to effect a down-shift to, say, the first gear ratio. While this may not pose a major problem when the vehicle is travelling at a low velocity, the abrupt down-shift to the first gear ratio, should it occur during a high vehicular velocity, would give rise to violent and unexpected engine braking that could subject the driver and passengers to a shock and lead to transmission or engine failure as well. Conventional failure detection and safety devices for precluding the foregoing mishap entail special circuit configurations and detection mechanisms and therefore tend to be complicated and expensive.